Wellbore Casing Integrity Envelope for Deepwater Reservoirs – Workflow and Case Study

Abstract

Abstract It has been known that offshore wellbores are prone to experience large deformation due to compaction of the reservoirs during the field production life. The induced compaction results in undesired effects on wellbore integrity and reservoir performance. Wellbore diameter restriction, casing shear, and cement de-bonding are a few examples of such effects. Permeability/porosity losses and fault reactivation are major concerns for the reservoir performance. It is the utmost goal for any operators to ensure long-term safety and integrity of their fields and wellbores for economical and HSE purposes. This demands a complex planning and modeling that a classical 1D geomechanical workflow cannot provide. Complex numerical simulation workflows coupled with adequate engineering planning and design are required to estimate safe operational characteristics given a field and wellbore conditions. Operators can characterize the safe operational envelope in terms of depletion and drawdown pressures since the reservoir compaction is highly dependent on these parameters. Several operators such as ExxonMobil have established their casing integrity evaluation throughout the lives of their fields either with analytical solutions, decoupled modeling by separating rock model from the casing behavior, or a sophisticated two-scale 3D finite element model (FEM) [3, 6]. It is believed that that the latter approach offers more in-depth understanding of the loading that may compromise casing integrity during production. This paper illustrates the necessary modeling components and workflow that is required to predict the safe depletion and drawdown pressure without compromising the integrity of a casing and re-activating any faults or fractures. ABAQUS 3D FE geomechanical modeling was coupled with the result of reservoir simulation to study drawdown and depletion effects. An FE submodeling technique was applied to account for formation, casing, and cement interactions. The technique will allow a fully coupled analysis using the boundary conditions established by the global (field-wide) 3D FE modeling. Any types of casing deformation modes (shear, tensile, buckling, and collapse) under in-situ stress changes due to depletion and drawdown, rock mechanical and fluid behaviors, and wellbore trajectory can be evaluated. In this paper, a deviated deepwater wellbore is analyzed using the Casing Integrity Envelope workflow. Two sections of a wellbore prone to casing deformations, one above and the other inside the reservoir, are examined. The resulting drawdown and depletion envelope enabled the operator to plan actions to mitigate well failures and to ensure safe operations. In general, the developed workflow can be created for any field, casing/cement configurations, and environment. It incorporates coupling of various materials with complex boundaries which are the main advantages over the traditional analytical models.